Patent Literature 1 discloses a fuel supply device. In this device, a fuel tank has an opening and a cap for covering the opening. The cap is used in order to install a component in the fuel tank, and a pillar-shaped member is installed between the cap and a bottom. Patent Literature 2 and Patent Literature 3 disclose examples of the component which should be installed in the fuel tank. Patent Literature 4 and Patent Literature 5 disclose snap-fit mechanisms in the fuel supply device. The content of Patent Literatures listed as prior art are used and incorporated by reference as description for technical components disclosed in this description.